GROWTH AND STRUCTURAL CHARACTERIZATION OF FE3O4 AND NIO THIN-FILMS AND SUPERLATTICES GROWN BY OXYGEN-PLASMA-ASSISTED MOLECULAR-BEAM EPITAXY

Oriented single-crystalline thin films of NiO and Fe3O4, and Fe3O4/NiO superlattices have been grown on cleaved and polished substrates of MgO(001), using oxygen-plasma-assisted molecular-beam epitaxy. We report the growth mode and structural characterization of the grown films using in situ reflection high-energy electron diffraction (RHEED) and ex situ scanning electron microscopy and x-ray diffraction. The (001) surface of MgO provides an excellent template for the pseudomorphic growth of these thin films and superlattices, for it has a very small lattice mismatch (0.3-0.9 %) to the cubic rock-salt structure of NiO and to the half unit-cell dimension of the spinel structure of Fe3O4. Superlattices consisting of alternating layers of NiO and Fe3O4 have been grown with a repeat wavelength down to 20 angstrom (approximately one Fe3O4 unit cell plus two NiO unit cells) thick. These superlattices exhibit strong crystalline ordering and sharp interface formation. RHEED pattern evolution in situ during growth indicates formation of the rocksalt NiO crystalline symmetry and then the spinel Fe3O4 crystalline symmetry in a periodic sequence as each material is being deposited. Our data indicate single-phase crystal growth in registry with the substrate, with films of overall cubic symmetry. Strain in the grown films exhibits interesting effects that clearly do not follow a simple elastic model.